The present invention relates to magnetic disk drives and magnetic disk media, and more particularly to magnetic disk drives, and the like, that are suitable for portable devices.
In recent years, with the development of small-size hard disk drives (HDDs), a personal digital assistant, or a cellular phone, has a built-in HDD in which video data and audio data (AV data) are accumulated. The video data and the audio data are reproduced and utilized on the road. The above is the tendency of products that are being developed. As for this portable device, the AV data is stored in the HDD that is a mass storage device; and at the time of reproduction, data is transferred, whenever necessary, from a magnetic disk medium to a buffer memory that is formed of a semiconductor memory. Thus, continuous reproduction is achieved by this buffer memory.
As far as portable devices for such uses are concerned, it is desirable that a period of time required to write AV data to a built-in HDD be as short as possible, and that the continuous reproduction time is as long as possible.
In recent years, HDDs can achieve a high medium transfer rate as a result of the speedup of the rotational speed. Here, an example taken as one of the conventional devices is a built-in HDD, a medium transfer rate of which is 4 MB/sec., and the start time from a stopped state to a steady state at high-speed rotation of which is 5 seconds. Accumulation/reproduction operation of music data and of image data using this example will be discussed as below.
First of all, for example, music data having a length of about 20 minutes, which is equivalent to three or four pieces of music, becomes MP3 data having a size of about 20 MB. The read time itself taken to read this amount of data is five seconds. The specified starting time described above is added to this. As a result, the driving time of the HDD is 10 seconds. To be more specific, the HDD consumes the electric power for acceleration for five seconds before the HDD enters a state of high-speed rotation, and then consumes the electric power to maintain the state of high-speed rotation for five seconds.
In addition, if MPEG-2 encoding is performed for content such as, for example, a movie having a length of two hours in consideration of the reproduction using a monitor having the normal size, the size of image data becomes about 4 GB. Moreover, an average reproduction rate of about 4 M bits per second is required. For example, on the assumption that the amount of data to be successively transferred from the HDD to the buffer is 20 MB, the driving time of the HDD is 10 seconds, which is the same as the above example of the music data. At the average reproduction rate described above, the length of time required to complete the reproduction of the data having a size of 20 MB is 40 seconds. To be more specific, during every 40 seconds, the electric power is consumed to start operation for five seconds and also to maintain the rotation for five seconds.